Since their discovery in 1993, single-walled carbon nanotubes (SWNT) have become an area of wide-reaching research and development activity due to their exceptional chemical and physical properties, including high strength, stiffness, and thermal and electrical conductivity. SWNT are hollow, tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons and pentagons. Single-wall carbon nanotubes typically have diameters in the range of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm.
Multi-wall carbon nanotubes are nested single-wall carbon cylinders and possess some properties similar to single-wall carbon nanotubes. However, since single-wall carbon nanotubes have fewer defects than multi-wall carbon nanotubes, single-wall carbon nanotubes are generally stronger and more conductive.
There is considerable interest in the chemical modification of single-wall carbon nanotubes to take advantage of single-wall carbon nanotubes' remarkable tubular framework structure in various applications, particularly, in the engineering of multi-functional materials. SWNT derivatized with organic functional groups can provide a high binding affinity and selectivity through formation of either hydrogen or covalent bonds. Through functionalization, SWNT can exhibit improved solubility in common organic solvents, as well as improved material properties and processability of composites, including fibers and nanotube-reinforced composite materials, such as those based on organic and inorganic polymers. SWNT that have been chemically derivatized with hydrophilic substituents, such as those containing terminal hydroxyl or carboxylic acid groups, are particularly attractive for medical and biological applications.
Exohedral SWNT functionalization can generally be classified into three main categories: 1) non-covalently bonded, supramolecular complexation, wrapping and coating with detergents and polymers, such as given in International Patent Publication, “Polymer-wrapped Single-wall Carbon Nanotubes” WO 02/016257 published Feb. 28, 2002, incorporated herein by reference in its entirety; 2) generation and functionalization of open and closed tube ends, such as given in International Patent Publication, “Carbon Fibers formed from Single-wall Carbon Nanotubes, WO 98/39250 published Sep. 11, 1998, incorporated herein by reference in its entirety; and 3) direct chemical functionalization of the nanotube sidewalls using addition reactions, such as given in “Chemical Derivatization of Single-Wall Carbon Nanotubes to Facilitate Solvation Thereof, and Use of Derivatized Nanotubes,” WO 00/17101 published Mar. 30, 2000, incorporated herein by reference in its entirety.
Of the two chemically-bonded functionalization categories, the functionalization referred to as “end-derivatization,” or “end-functionalized” will be defined herein to include bonds on edges and open tube ends. “Sidewall derivatization” or “sidewall functionalization” will be defined herein to include bonds made to the wall that keep the carbon-carbon bonds of the wall intact. “End-cap derivatization” or “end-cap functionalization” will be defined herein to include bonds made to the end cap that keep the carbon-carbon bonds of the end cap intact. Regardless of whether the single-wall carbon nanotubes are derivatized on their ends, sides, end caps, or combination thereof, the SWNT will be referred to as derivatized SWNT for convenience and clarity.
End-functionalization of single-wall carbon nanotubes often proceeds through oxidation routes to form shortened nanotubes with carboxylic acid groups at the tube ends which can be further derivatized by reactions with a chlorinating agent, such as thionyl chloride, and long-chain amines or by esterification. Carboxylic acid functionality can also be created on SWNT edges and defects on partially etched (“unzipped”) side walls by oxidative treatment with various oxidants.
Sidewall functionalization of carbon nanotubes, in which the nanotube walls are kept intact, has been much more difficult to achieve than open end-functionalization. Methods to functionalize SWNT sidewalls with organic groups include fluorination (see E. T. Mickelson, et al., Chem. Phys. Lett. 1998, 296, 188), followed by subsequent reactions with reactions with alkyl lithium and metal alkoxides (see P. J. Boul, et al,. Chem. Phys. Lett. 1999, 310, 367, R. K. Saini, et al., J. Am. Chem. Soc. 2003, 125, 3617, and E. T. Mickelson, et al., J. Phys. Chem. B, 1999, 103, 4318–4322), as well as by Grignard reagents (see V. N. Khabashesku, et al, Acc. Chem. Res. 2002, 35, 1087, and Khabashesku, V. N. and Margrave, J. L. “Chemistry of Carbon Nanotubes” in The Encyclopedia of Nanoscience and Nanotechnology, S. Nalwa, Ed. American Scientific Publ. 2003) or diamines (see J. L. Stevens, et al., Nano Lett. 2003, 3. 331) and reactions with aryl diazonium salts (see J. L. Bahr, et. al., J. Am. Chem. Soc. 2001, 123, 6536–6542 “Bahr”), azomethine ylides (see V. Georgakilas, et al., J. Am. Chem. Soc. 2002, 124, 760; V. Georgakilas, et al., J. Chem. Soc. Chem. Commun. 2002, 3050; D. Pantarotto, et al., J. Am. Chem. Soc. 2003, 125, 6160); carbenes (see Y. Chen, et al., J. Mat. Res. 1998, 13, 2423–2431, J. Chen,. et al., Science, 1998, 282, 95–98, and M. Holzinger, et al., Angew. Chem. Int. Ed 2001, 40, 4002–4005 (“Holzinger”)); nitrenes (see Holzinger) and organic radicals (see Holzinger, H. Peng, et al, J. Chem. Soc. Chem. Commun., 2003, 362, and Y. Ying, et al., Org. Lett. 2003, 9, 1471).
One method of functionalizing fullerenes with moieties having terminal carboxylic acid groups has been demonstrated with C60 using a two-step process (Bingel 2+1 cycloaddition reaction followed by deesterification) yielding carboxylated methanofullerene structures. (see Kini, V. U.; Khabashesku, V. N.; Margrave, J. L. Rice Quantum Institute Sixteenth Annual Summer Research Colloquium. Aug. 9, 2002, Abstr. p. 25.) However, when applied to single-wall carbon nanotubes, the process was much less efficient due to the inertness of single-wall carbon nanotube to carbene addition via Bingel-type reaction.
Sidewall functionalization of single-wall carbon nanotubes with aryl radicals has been reported when aryl diazonium salts were reduced electrochemically using single-wall carbon nanotube buckypaper as electrodes. (see Bahr). Functionalization has also been reported using diazonium compounds generated in situ. (see J. L. Bahr, et al., Chem. Mater. 2001, 13, 3823–3824). Radical addition of perfluoroalkyl groups generated by photolysis of corresponding species possessing a carbon-iodine bond has also been reported by Holzinger. Other examples of sidewall functionalization include electrochemical reductive and oxidative coupling by substituted phenylated derivatives (see S. E. Kooi, et al., Angew. Chem. Int. Ed, 2002, 41, 1353–1355) and electrophilic addition of chloroform followed by hydrolysis and esterification (see N. Tagmatarchis, et al., Chem. Commun., 2002, 2010–2011). Dissolved lithium metal in liquid ammonia (Birch reduction) was used to hydrogenate SWNT. (see S. Pekker, et al., J. Phys. Chem. B., 2001, 105, 7938–7943.)
There remains, however, a need for a convenient and efficient method for non-destructively functionalizing single-wall carbon nanotubes with a variety of functional groups, especially organic groups which can be used for further reactions, so as to be bound or otherwise associated with polymers, biomedical species, and other materials for a particular end-use application.